The present invention relates to a liquid crystal display device, and more particularly, it relates to a liquid crystal display device having a wide viewing angle characteristic and high display quality.
Recently, a thin and light liquid crystal display device is used as a display device for a display of a personal computer or a display unit of portable information terminal equipment. Conventional twist nematic (TN) or super twist nematic (STN) liquid crystal display devices have, however, a disadvantage of a narrow viewing angle, and a variety of techniques have been developed for overcoming this disadvantage.
A typical technique to improve the viewing angle characteristic of a TN or STN liquid crystal display device is a method of additionally providing an optical compensator. Another technique is a lateral field method of applying, through a liquid crystal layer, an electric field in a lateral direction to the substrate surface. Liquid crystal display devices of the lateral field method are recently mass-produced and regarded as promising devices. A still another technique is DAP (deformation of vertical aligned phase) in which a nematic liquid crystal material with negative dielectric anisotropy is used as a liquid crystal material and a vertical alignment film is used as an alignment film. The DAP is a kind of an electrically control birefringence (ECB) method, in which the transmittance is controlled by utilizing the birefringent property of the liquid crystal molecules.
Although the lateral field method is one of the effective methods for attaining a wide viewing angle, the production margin is very small in the production process as compared with that of a general TN liquid crystal display device, and hence, there is a difficulty in stable production of this type of liquid crystal display devices. This is because gap irregularity between substrates and shift of the transmission axis of a polarizing plate (polarization axis) from the orientation axis of a liquid crystal molecule largely affect the luminance and the contrast ratio of display. In order to stably produce the liquid crystal display devices of the lateral field method by highly precisely controlling these factors, the technique should be further highly developed.
Furthermore, in order to produce an even display free from display unevenness by a liquid crystal display device of the DAP method, it is necessary to control orientation. For controlling the orientation, an alignment treatment is carried out by rubbing the surface of an alignment film. When the surface of a vertical alignment film is subjected to a rubbing treatment, however, rubbing streaks are easily caused in a displayed image. Therefore, this treatment is not suitable to mass-production.
On the other hand, for controlling the orientation without the rubbing treatment, a method for controlling the orientation directions of liquid crystal molecules by an inclined electric field generated by forming a slit (opening) in an electrode has been proposed (as described in, for example, Japanese Laid-Open Patent Publication Nos. 6-301036 and 2000-47217). However, the present inventors have found the following as a result of examination: The orientation state of a portion of a liquid crystal layer corresponding to the opening of the electrode is not specified in the methods disclosed in these publications, and the continuity of the orientation of the liquid crystal molecules is not sufficient. Therefore, it is difficult to obtain a stable orientation state over an entire picture element, and hence, a displayed image becomes disadvantageously uneven.